1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus for forming an image on a recording medium by using an ink containing coloring material and resin particles in a solvent and a treatment liquid for aggregating the coloring material.
2. Description of the Related Art
An inkjet recording system performs recording by ejecting and depositing droplets of ink onto a recording medium from a plurality of nozzles formed in an inkjet head, and such a system is able to record images of high resolution and high quality, with little noise during the recording operation and low running costs. The ink ejection system may be, for example, a piezoelectric system, which uses the displacement of a piezoelectric element, a thermal system, which uses thermal energy generated by a heating element, or the like.
In the inkjet recording system, when ink droplets are consecutively deposited in such a manner that the ink droplets (ink dots) that are mutually adjacent on the recording medium overlap with each other, these ink droplets combine together due to their surface tension and give rise to a problem of bleeding (landing interference) in which the desired dots cannot be formed. In the case of dots of the same color, the dots shape is disturbed and in the case of dots of different colors, an additional problem of color mixing occurs. In particular, when recording with a single-pass system using a line head, the difference in the landing time between mutually adjacent ink droplets is short and therefore landing interference is liable to occur and it is difficult to form a sharply defined image.
In response to this, technology is known which achieves high image quality by depositing a so-called treatment liquid onto a recording medium prior to the ink liquid, and causing this treatment liquid to react with the ink. When using pigment particles as the coloring material, the treatment liquid has the function of aggregating the pigment particles by neutralizing the Coulomb repulsion of the particles and thereby increasing the viscosity of the ink liquid. Thereby, interference between deposited dots is suppressed and sharply defined images can be recorded without the occurrence of non-uniformities in density.
Moreover, technology is also known in which thermoplastic resin particles (polymer particles) are added to ink in order to impart a suitable luster to the formed image or to achieve good adhesiveness with the recording medium. If thermoplastic resin particles are added, then by selecting a suitable particle size and dispersant, it is possible to raise the speed of aggregation, which is beneficial for recording sharply defined images.
Furthermore, in a high-seed printing method of this kind, it is necessary to dry the printed ink, and if this drying is not sufficient, then problems of movement of the ink or blocking (sticking of the recording medium) may occur when the recording medium is outputted. Moreover, in particular when a water-based ink is used, there is a large problem of curl occurring in the recording medium due to insufficient drying. In response to this, technology is known for resolving these problems by carrying out heating and drying after depositing ink on the recording medium.
Japanese Patent Application Publication No. 2007-160839 discloses heating and drying the recording medium within three seconds after depositing droplets of ink. By this means, it is possible to prevent curl and improve the print density.
Japanese Patent Application Publication No. 2003-048317 discloses depositing a fixing agent that fixes the ink onto the recording medium, heating and drying the undercoating liquid, depositing ink, depositing a fixing agent onto the recording medium, and heating and drying the ink layer.
PCT Publication No. WO 94/01283 discloses an inkjet recording apparatus of an intermediate transfer type, in which ink containing a thermoplastic resin is deposited onto an intermediate transfer medium, the ink is heated to the softening point of the resin or higher, and the ink is transferred onto a recording medium.
However, it has become clear that in a case where an aggregating treatment liquid is deposited onto the recording medium, droplets of an ink containing thermoplastic resin particles are deposited onto the recording medium and the ink film formed on the recording medium is heated and dried, there is problem that the thermoplastic resin particles on the recording medium fuse due to the effects of heating and drying and thus give rise to contraction and deformation of the image.
FIG. 14A shows a satisfactory image that has been formed by drying the ink film at room temperature, and FIG. 14B shows an image in which deformation has occurred due to heating of the ink film. The images in these examples are images of white characters on a black background, in which the contraction of the ink film surrounding the characters produces image deformation which appears as thickening of the characters.